By the turn of the century the field of automatic process control had become an area of concentrated scientific research. The idea of using machines rather than people to monitor automatic processes was attractive to businessmen seeking to streamline their operations. Automatic monitors could operate continuously, at a low cost, and they could be equipped to rapidly respond to changing conditions in the monitored process. These machines were "intelligent" in the sense that they employed logical elements to "make decisions" about the state of the process based on information received through process sensors. After evaluating the information, an automatic monitor could set the state of the monitored process according to some preset sequence of instructions. The earliest automatic process controllers were limited to one preset response. However, as the logical elements of the monitors evolved adaptive responses became possible.
The logical elements of automatic process controllers evolved steadily from their modest beginnings to the current state of the art. They progressed from electro-mechanical devices and pneumatics, to electron tubes, and eventually, to logic gates composed of multiple transistors. The advent of transistors quickly led to the use of integrated circuits and, in turn, to the use of microprocessors. By the late 1960's the computer was introduced as the main logic element of automatic process controllers.
The use of microprocessors and computers as the main logic elements of automatic process controllers gave designers the freedom to implement complex control processes which could respond to any number of problems within the monitored process. One problem that designers set out to solve was that of continuity of the process. This problem arises when a critical element within the control loop begins to malfunction, giving rise to an error indication and corresponding process shutdown. Such a shutdown could have disastrous consequences. For example, the dire results of a failure in a critical element of an aircraft's fuel supply system which caused the process controller to direct a shutdown of the entire fuel system, without a backup mode in place, are obvious. To solve continuity of process problems such as this, designers made use of microprocessor controlled logic elements to implement "fail safe" designs that would avoid process shutdowns.
Automatic process controllers began to incorporate systems that would maintain the process in a static condition upon the occurrence of a failure. An example of such a fail safe design is the crash avoidance mechanism present in many of today's jet fighter planes. The planes are equipped with sensors capable of detecting the pilot's loss of consciousness or "blackout"--an event which may occur when the pilot is executing high g force maneuvers. In the event that a "blackout" is detected the plane's control surfaces are adjusted via computer to maintain straight and level flight. In this manner, a blackout will not result in the loss of the plane and pilot. While fault tolerant systems which can maintain processes in a static condition offer many advantages, even greater advantages are realizable by systems that capable of implementing one of several static conditions.
Fault response systems which maintain a process in a static condition are usually hardwired. This makes them difficult to reconfigure without burdensome re-wiring. Thus, they are inflexible and can not be adapted to changing conditions in the monitored process.
In overcoming the disadvantages of prior-art process controllers, the present invention provides easily reconfigurable fault response circuitry. The invention allows the fault response of the system to be varied either by a human operator or through additional logical elements, thereby providing a highly flexible fault response.